1. Field of the Invention
The invention relates to an image sensor and a method therefor, and more particularly to an image sensor including at least two sets of photosensors composed of a red-light photosensor, a green-light photosensor and a blue-light photosensor to enhance the resolution in the moving direction of the image sensor, and a method therefor.
2. Description of the Related Art
FIG. 1A is a schematic illustration showing an optical path in a conventional scanner for scanning a document. Taking a reflective type of document as an example, a typical scanner acquires an image by projecting light rays L from a lamp 110 onto a to-be-scanned document 112. Then, the light rays L enter the optical module 113 and is reflected by a reflecting mirror 114, focused by a lens module 116, and then received by a CCD (charge coupled device) module 118, which converts optical signals into electric signals to be transferred to a circuit board 120. Then, a stepping motor (not shown) drives an optical module 113 to sequentially move along the Y-axis until the whole to-be-scanned document 112 is scanned. The image sensing resolution for the CCD module 118 is one important factor for determining the quality of the scanned image.
FIG. 1B is a schematic illustration showing the optical paths for the CCD module 118 of FIG. 1A to read the image of the to-be-scanned document 112. The longitudinal CCD module 118 may read a scan line 122 on a to-be-scanned document 112 in the X-axis direction. The resolution in the X-axis direction is referred to as an optical resolution, and “dpi (dots per inch)” may be used to represent the number of sensing cells (typically photodiodes) in the CCD module 118, receiving light rays reflected from each inch of document. In addition, because the stepping motor drives the optical module 113 to move in the Y-axis direction, the CCD module 118 may read a plurality of scan lines 122 of the to-be-scanned document 112, and the resolution in the Y-axis direction is referred to as a mechanical resolution, which represents the number of steps of the stepping motor for moving the CCD module 118 by one inch, or the number of scan lines read when the CCD module 118 is moved by one inch in the Y-axis direction. Therefore, the resolution of the scanner is generally represented by (X-axis resolution×Y-axis resolution) dpi, such as 600×300 dpi.
Furthermore, please refer to FIG. 1C, which is a schematic illustration showing the optical paths for the conventional CCD module to read the to-be-scanned document using three CCDs to filter the red-light (R), green-light (G) and blue-light (B). The typical CCD module 118 further includes RCCD, GCCD and BCCD for sensing red-light, green-light and blue-light components, respectively. The three CCDs and the corresponding lens module 116 are arranged such that the RCCD, GCCD and BCCD may read three adjacent scan lines 122, such as scan lines Ln, Ln+1 and Ln+2 of FIG. 1C, of the to-be-scanned document 112 at a time. When the stepping motor drives the CCD module 118 to move along the Y-axis to the next step for reading the next scan line, the RCCD, GCCD and BCCD (dashed lines in FIG. 1C) may read the next three adjacent scan lines Ln+1, Ln+2 and Ln+3 at a time, and the processes may be performed analogically, as shown in Table 1. At time Ti, the reflected light rays L from three adjacent scan lines Ln, Ln+1 and Ln+2 may enter RCCD, GCCD and BCCD, respectively. When the CCD module 118 is moved to a next step at time Ti+1, the reflected light rays L from the three subsequent adjacent scan lines Ln+1, Ln+2 and Ln+3 may further enter RCCD, GCCD and BCCD, respectively, and the processes are performed analogically. It can be understood that the image of the same scan line Ln+2 may be read by the RCCD, GCCD and BCCD at time Ti, Ti+1 and Ti+2, respectively. Similarly, other scan lines Ln+3, Ln+4 . . . may also be read in the same manner. The three primary colors of R, G, B may be mixed to form colored light rays, and a complete colored image may be obtained accordingly.
TABLE 1
With the progress of the technology and users' demands on the higher colored image quality, manufactures of scanners have been continuing improving the resolutions of their newly developed products. As described above, the scanning resolution of the scanner, with respect to the document, depends on the resolutions in both of the X-axis and Y-axis directions. Thus, both resolutions, in the X-axis direction and in the Y-axis direction, have to be enhanced. However, typically the resolution in the X-axis direction is enhanced by increasing the number of sensing cells contained in each of the three CCDs (RCCD, GCCD, BCCD), and the resolution in the Y-axis direction is enhanced by an improved stepping motor and an improved lamp 110.
For example, a scanner for scanning 8″×11″ documents at a scanning speed of 30 ppm will be described. In other words, the scanner needs only two seconds to scan a page of document having a length of 11 inches. The stepping motor for the scanner with the Y-axis resolution of 300 dpi has a transmission frequency of 3300/2 (steps/seconds) because 3300 scan lines have to be scanned, and the holding time (scan time) in each step is equal to 2/3300 (seconds/steps). Consequently, the exposure time of the sensing cells is equal to 2/3300 seconds. When the resolution is enhanced to 600 dpi while at the constant scanning speed of 30 ppm, the transmission frequency for the stepping motor should be changed to 6600/2 (steps/seconds) because the number of scan lines in each page is increased to 6600. That is, the rotating speed of the stepping motor has to be doubled. At this time, the scan time per scan line is shortened to 1/3300 (second/steps). That is, the exposure time of the sensing cells is shortened to 1/3300 (seconds). However, the typical maximum torque T is exponentially inversely proportional to the rotating speed u(rpm) of the stepping motor, as shown in the curve of FIG. 1D. When the rotating speed of the motor increases from, for example, uo to ui, the maximum torque that can be produced by the motor decreases from To to Ti. At this time, the stepping motor may not stably drive the scanning module, and a larger or a more expensive stepping motor has to be used. In addition, when the holding time of the stepping motor is shortened, the exposure time for the sensing cells is also shortened relatively. Thus, the brightness of the lamp 110 has to be increased by, for example, increasing the number of lamps or applying a high voltage to the lamp, which economically increases the cost of production or shortens the lifetime of the lamp.
The above-mentioned scanner is an example of flatbed scanner where the to-be-scanned document 112 is stationary and placed on a scan platen and the stepping motor drives the optical module 113 to move in the Y-axis direction. The same problem also exists in a sheet-fed scanner where the optical module 113 is stationary and the stepping motor drives the to-be-scanned document 112 to move forward because a relative movement between the to-be-scanned document 112 and the optical module 113 also exists. For the sake of clarification, the invention only illustrates the scan mode of the flatbed scanner, but one of ordinary skill in the art may understand that the invention also applies to the scan mode of the sheet-fed scanner.